Wire temperature measuring method for a wire electric discharge machine

ABSTRACT

A wire temperature measuring method for a wire electric discharge machine by which temperature and temperature distribution of a wire can be accurately measured during electric discharge machining. After a machining voltage is cut off, a loop current flows. Based on the loop current and a voltage between electrodes measured by a differential amplifier, a resistance between the electrodes is obtained, and the temperature of a wire is obtained based on the resistance. Accordingly, the wire temperature during actual machining operation can be measured with accuracy.

TECHNICAL FIELD

The present invention relates to a wire temperature measuring method fora wire electric discharge machine, and more particularly, to a wiretemperature measuring method for measuring the temperature distributionof a wire in a wire electric discharge machine.

BACKGROUND ART

The machining speed of a wire electric discharge machine is greatlyaffected by wire disconnection. Namely, precise machining cannot beeffected under conditions in which the frequency of wire cut is high,and accordingly, a machining operation has to be carried out underconditions (low cutting speed, low discharge current)less severe thansuch machining conditions.

Generally, wire disconnection is considered to be caused because thewire temperature locally rises and thus the wire strength lowers there,as a result of concentrated electric discharge during the machiningoperation, and it has been confirmed that after concentrated electricdischarge actually occurs at a portion of a wire, the wire isdisconnected at that portion. Therefore, efforts have been made to avoidconcentrated electric discharge, for example, to control the on-off timeof a machining power supply.

The most important factor in the phenomenon of wire disconnection isconsidered to be the wire temperature and the temperature distributionduring a machining operation. Specifically, if the local temperaturerise of the wire is moderate during machining operation, then it isnecessary that the entire wire be cooled more intensely. Conversely, ifthe local temperature rise of the wire is extremely high, it isnecessary to distribute the electric discharge. In any event, it is atpresent difficult to directly measure the wire temperature, andeffective measures still remain unfound.

A first method currently implemented for measuring the wire temperatureis a method in which the wire temperature is estimated based on thetemperature-strength characteristic of the wire. When the wire isdisconnected under certain conditions, the temperature of thedisconnected portion of the wire is estimated from the tension thenacting on the wire.

A second method is a method based on analysis, wherein the wiretemperature is calculated based on an assumed random heating of the wirealong the length thereof and an assumed heat transfer coefficient of awire surface.

According to the first method, however, only the temperature of thedisconnected portion of the wire is estimated, and it is not possible tofind the temperature of the entire wire or the temperature distribution.

The second method does not take account of the effect produced by theflow of cooling water in the machining groove, which provides a dominantwire cooling effect. Thus, estimation of the heat transfer coefficientis not as effective as expected, and actual wire temperature cannot becalculated with high accuracy.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above circumstances,and an object thereof is to provide a wire temperature measuring methodfor a wire electric discharge machine by which accurate measurement ofwire temperature can be permitted.

Another object of the present invention is to provide a wire temperaturemeasuring method for a wire electric discharge machine by whichtemperature distribution can be accurately measured.

To achieve the above objects, the present invention provides a method ofmeasuring the temperature of a wire during a machining operation of awire electric discharge machine, wherein a resistance value of the wireis obtained based on a loop current flowing after cutoff of a dischargevoltage, and the temperature of the wire is obtained based on theresistance value.

The present invention also provides a method of measuring thetemperature of a wire during a machining operation of a wire electricdischarge machine, wherein an electric discharge position is detectedbased on upper and lower currents flowing, respectively, from upper andlower electrodes during electric discharge, a resistance value betweenthe electric discharge position and a lower end of a workpiece ismeasured based on the upper current, the lower current, and a loopcurrent flowing after cutoff of a discharge voltage, and temperaturedistribution of the wire is measured based on a plurality of theresistance values.

When the discharge voltage is cut off, a loop current flows due toimpedance mismatching of the wire. The resistance of the wire iscalculated based on the loop current, and the wire temperature ismeasured based on this resistance.

Moreover, the electric discharge position is detected based on the upperand lower currents. More specifically, the upper current has a largevalue if the electric discharge position is displaced upward, and thelower current has a large value if the electric discharge position isdisplaced downward. Based on the electric discharge position and theloop current, the resistance of the wire between the electric dischargeposition and the lower end of a workpiece is obtained. This resistanceis obtained a plurality of times, whereby the temperature distributionof the wire can be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a measuring circuit formeasuring the temperature of a wire; and

FIG. 2 is a circuit diagram illustrating a measuring circuit formeasuring the temperature distribution of a wire.

BEST MODE OF CARRYING OUT THE INVENTION

An embodiment of the present invention will be now described withreference to the drawings.

FIG. 1 illustrates a measuring circuit for measuring the temperature ofa wire. A wire 1 is supplied with a discharge voltage from a machiningpower supply 5 via feeders 2 and 3, and the discharge voltage is appliedbetween the wire 1 and a workpiece 4. The wire 1 is accuratelypositioned by means of upper and lower guides 6 and 7. After thedischarge voltage is cut off, a loop current I_(L) flows due toimpedance mismatching of upper and lower feeder lines 8a and 8b.

To measure the loop current I_(L), voltage detection electrodes 11 and12 are arranged at the wire 1 in the vicinity of upper and lower sides,respectively, of the workpiece 4. The electrodes 11 and 12 each have athin annular shape surrounding the wire 1 so as not to obstruct the flowof water from upper and lower nozzles, not shown. Accordingly, the wirevoltage is measured based on the leakage current flowing through thewater. Accurate voltage measurement is possible on condition that thepolarization voltage and resistance between the wire 1 and each of theelectrodes 11 and 12 are sufficiently low compared with the detectedvoltage and the resistance of the electrodes 11 and 12.

If a direct current is passed between the wire 1 and the electrodes 11,12 (leakage current flowing through the water), measurement error occursdue to oxidation of the surfaces of the electrodes 11 and 12 or airbubbles produced; therefore, an AC voltage is detected by means of acapacitor coupling including two capacitor-resistor pairs, i.e., C1-R1and C2-R2. The differential of the voltage, i.e., the potentialdifference between the electrodes 11 and 12, is measured by adifferential amplifier 13.

Generally, the loop current I_(L) remains constant under fixed machiningconditions. Accordingly, it is possible to calculate the resistance ofthe wire 1 between the electrodes 11 and 12 based on the measuredvoltage and the loop current I_(L), and thus to obtain the wiretemperature based on the calculated resistance.

Although in the above embodiment, the measuring circuit for measuringthe loop current includes a capacitor coupling, it may be constituted bya transformer coupling.

FIG. 2 illustrates a measuring circuit for measuring the temperaturedistribution of the wire. In FIG. 2, like reference numerals are used todenote like elements appearing in FIG. 1, and a description of suchelements is omitted. As shown in FIG. 2, a pair of contactors 21 and 22is arranged between the feeders 2 and 3, and the voltage between thecontactors 21 and 22 is measured. Provided that currents I_(u) and I_(d)flow from the feeders 2 and 3, respectively, during electric discharge,the voltage V between the contactors 21 and 22 is given by

    V=-I.sub.u (r.sub.u +R-r.sub.i)+I.sub.d (r.sub.d +r.sub.i) (1)

where

r_(u) : the resistance of the wire between the upper end of theworkpiece 4 and the contactor 21;

R: the resistance of the wire between the upper and lower ends of theworkpiece 4;

r_(i) : the resistance of the wire between the lower end of theworkpiece 4 and an electric discharge point P; and

r_(d) : the resistance of the wire between the lower end of theworkpiece and the contactor 22.

The position P of the electric discharge can be determined from thevalue of the current flowing in the vertical direction in the figure.

Using the suffixes n, 1 and i for the discharge at the upper end of theworkpiece 4, the discharge at the lower end of the workpiece, and thedischarge midway between the upper and lower ends, respectively, thenthe following equations are derived:

    V.sub.n =-I.sub.un r.sub.u +I.sub.dn (r.sub.d +R)          (2a)

    V.sub.l =-I.sub.ul (r.sub.u +R)+I.sub.dl r.sub.d           (2b)

    V.sub.i =-I.sub.ui (r.sub.u +R-r.sub.i)+I.sub.d (r.sub.d +r.sub.i)(2c)

While the equations are three in number, there are four unknowns r_(u),R, r_(i) and r_(d), and thus it is not possible to find these unknowns.Therefore, the loop current I_(L) flowing when the machining voltage iscut off, and the voltage V_(L) then developed between the contactors 21and 22 are measured. This measurement is carried out by a differentialamplifier 23 connected to the contactors 21 and 22. Accordingly, theresistance R_(O) between the contactors 21 and 22 can be obtained by thefollowing equation:

    R.sub.O =r.sub.u +R+r.sub.d =-V.sub.L /I.sub.L             (3)

Therefore, from the equations (2a) to (2c) and (3), r_(u), R, r_(i) andr_(d) can be obtained from the following equations:

    r.sub.u =(R.sub.O I.sub.dn -V.sub.n)/(I.sub.un +I.sub.dn)

    r.sub.d =-(R.sub.O I.sub.dl +V.sub.l)/(I.sub.ul +I.sub.dl)

    R=R.sub.O -r.sub.u -r.sub.d

    r.sub.i =[V.sub.i +I.sub.ui (R+r.sub.u)-I.sub.di r.sub.d ]/(I.sub.ui +I.sub.di)

If r_(i) is found, then the resistance of the lower end of the workpiece4 can be obtained. Accordingly, by repeating this measurement, it ispossible to measure the temperatures at a multiplicity of points. Themeasured values are differentiated with respect to distance, whereby thetemperature distribution per unit length can be obtained.

The result of the measurement by means of the aforementioned measuringcircuit revealed that the temperature of the wire rose up to about 100°C. Since the temperature at which the wire is disconnected isapproximately 300°to 400° C., it is considered that wire disconnectionis caused by concentrated electric discharge.

As described above, according to the present invention, the temperatureof the wire is measured based on the loop current flowing after cutoffof the machining voltage, whereby the wire temperature can be accuratelymeasured with a simple arrangement.

Further, since the electric discharge position and the currentdistribution are measured based on the loop current and the upper andlower currents flowing from the upper and lower feeders, respectively,the temperature distribution of the wire can be measured with accuracy.

We claim:
 1. A method of measuring the temperature of a wire during amachining operation of a wire electric discharge machine on a workpiece,comprising:supplying a discharge voltage to the wire via first andsecond feeders which position the wire, the first and second feedersconnected to respective first and second feeder lines; cutting off thedischarge voltage; obtaining a resistance value of the wire based on aloop current flowing, after the cutoff of the discharge voltage, as aresult of impedance mismatching of the first and second feeder lines;and obtaining the temperature of the wire based on the resistance value.2. The method as claimed in claim 1, wherein said step of obtaining aresistance value further comprises:arranging voltage detectionelectrodes at respective first and second positions in the vicinities ofrespective first and second sides of the workpiece past which the wireruns; and measuring a voltage difference of the wire between the firstand second positions to determine the loop current.
 3. The method asclaimed in claim 2, wherein said step of measuring the voltagedifference of the wire between the first and second positionscomprises:detecting AC voltages at the respective voltage detectionelectrodes; and taking a difference between the AC voltages to determinethe voltage difference.
 4. The method as claimed in claim 2, whereinsaid step of measuring the voltage difference of the wire between thefirst and second positions comprises:detecting AC voltages at therespective voltage detection electrodes using capacitor-resistor pairs;and taking a difference between the AC voltages to determine the voltagedifference using a differential amplifier.